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What is CD-ROM?

The term CD-ROM stands for Compact Disc Read Only Memory.
Its "ROM" part, however, is rather misleading since the "M", short for Memory, usually does not apply to magnetic storage devices like floppy or hard disks. As it stands for, "ROM" is a memory device that reads data but cannot write them back. In general, it is applied to the storage devices made of IC chips like semiconductors. Of course, the "ROM" in CD-ROM is not a totally incorrect term; since you cannot write data on CD-ROMs after all.
CD-ROM, a high-capacity data storage system, is an extension to the Red Book CD format, or CD-DA (Compact Disc-Digital Audio). The CD-DA were for audio data only; yet, CD-ROM is a standardized format not only for audio data but for computer data (Yellow Book).
CD-ROM Capacity
MODE 1 : 650 MBytes
MODE 2 / FORM 1 : 650 MBytes
MODE 2 / FORM 2 : 738 MBytes
The capacity is closely related to the track density of the disc. When high-density optical disc drive is developed based on blue laser technology, the capacity could amount to more than 10 gigabytes. (As of this writing, a single side/single layer DVD has a capacity of 4.7 bytes)

CD-ROM Basics

● Major Physical Specifications of CD/CD-ROM
Classification
Standard
Play Time (Single Side, 2-Channel)
about 60min
Scanning Velocity (2-Channel)
1.2 ~ 1.4 m/s
Rotating Direction (Read Side)
counter clockwise
Track Pitch
1.6um
Disc Diameter
120mm
Disc Thickness
1.2mm
Center Hall Diameter
15mm
Starting Diameter of Program Area
50mm
Number of Channel
2 or 4 (Simultaneously sampled)
Sampling Frequency
44.1KHz
Quantization
16bit linear/channel, 2's complement
Channel bit rate
4.3218Mb/s
Pre-Emphasis
no or 50/15us

CD-ROM/CD-DA Logical Specifications

Determinant of data size on CD-ROM: Sector
1) Data is stored by the unit of "sector", or "frame."
(Logical sector size: 2,352 bytes) - Size of user data area differs depending on the sector mode engaged at the time of data storage.
2) Number of sectors
CD capacity is measured by the unit of time (MSF). Of the total 74 minutes, the last 14 minutes (5 mm from the outside) is generally excluded when measuring the capacity.
3) CD-players play back at the rate of 75 sectors per minute, and
Mode 1 contains 2,048 bytes of user data, thus;
2,048 Byte × 75 Sector × 60 sec × 60 min. = 552,966,000 Byte
Mode 2 contains 2,336 bytes of user data, thus;
2,336 Byte × 75 Sector × 60sec × 60min. = 630,720,000 Byte

Basic unit of data on a CD:14 bits

(data processing unit on a computer: 8 bits)
When storing information on discs
EFM (Eight to Fourteen Modulation) is a method of encoding source data into 4 bit code by use of the Look Up Table. With EFM, 8-bit data bytes are converted into 14 bit units that always have a minimum of 2 "zeros" between "ones."
LUT(Look Up Table) : A reference table used in converting 8 bits of data into 14 bit code.
Sending disc data to a decoder
It works in the opposite direction of EFM. Disc data are converted into 8-bit information, which then are sent to the system. Channel bit data on discs are sent to the decoder through transmission channel. This conversion is done in hardware using a ROM lookup table.
(ex) RL (Run Length) : Length of Pits and Lands
(Maximum RL: 11 channel bits / Minimum RL: 3 channel bits)

CD-ROM Interface

AT-BUS :
All of data processing takes place inside the CPU
SCSI :
The adapter helps data processing, thus reducing the level of CPU overload. As a result, the overall performance of the system is improved.
note
Disk/Disc
Disk" : Floppy and hard 'disks'
Disc" : Optical 'discs' like CD or CD-ROM.
CD-Audio : CD-Audio discs were first introduced to the market in 1982. Convenient to take care
  of and with quality sound, the discs began to replace LPs on the market at a rapid rate.
 
Difference between CD-Audio and CD-ROM
: They look identical, yet don't have much in common actually.
When a CD-ROM disc is played back by an audio CD player, the player, which can't read the disc, would only make funny noises.
CD-Audio : Mostly audio data (sometimes graphic)
CD-ROM : text, images, graphics, sound, and other information in digital form. CD-ROM and
   CD-Audio are used in fundamentally different ways.
CD-ROM wouldn't replace floppy or hard disks, yet its varied features are a strong indicator
that CD-ROM will be widely used..
Due to its high-capacity storage, size and preservability, CD-ROM is a highly efficient
storage device, especially for a big application package. However, while floppy or hard disks can both be read or be written on, CD-ROMs are like books that can only be read. So, it's an easy guess that CD-RW will be in high demand in due time.

CD-ROM & CD Audio (CD-Player)

1 CD Audio
The standard method employed in the CD Audio format is called Pulse Code Modulation (PCM), which transforms the analog signal into digital one.
Process of CD Audio Storage
Sampling : It is to divide sound wave (analog signal) into samples. Sampling rate of 44.1 KHz
means that the analog signal is divided into 44,100 samples per second.
CD standard audio uses 16-bit quantization thus giving 65536 values.
Each sample is decoded into the binary code of 0s and 1s. In the process,
it takes up Cross Interleaved Reed Solomon Code (CIRC) for error detection and correction before it finally gets stored on CD.

CD-ROM (Text/Sound/Graphic/Animation/Video Data)

CD-ROM Data Storage
Besides CIRC, CD-ROM contains Cyclic Redundancy Check (CRC), Error Detection Code (EDC)
and Error Correction Code (ECC), all of which are related to dealing with errors. Due to EDC and ECC, for example, data reading errors which might occur because of small scratches on the disc surface can be corrected, and the data would be read accurately.
 
 
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